The invention relates to an electropneumatic control valve for a pneumatic brake system of a vehicle.
The electropneumatic valve of the present invention has a relay valve housing in which an axially adjustable control piston is arranged for actuating a valve seat arrangement which switches the compressed-air flow between at least one exterior brake line connection, a supply pressure connection and a bleeder connection. The control piston of the control valve is axially guided by means of a guiding arrangement and can be acted upon by a control pressure by way of at least one electromagnetic pilot valve for the axial adjustment. The pilot valve is accommodated in a pilot valve housing connected with the relay valve housing. The present invention specifically relates to a special constructive design of the guiding arrangement of the control piston.
A control valve of the above-mentioned type is used within a pneumatic brake system of a vehicle in order to control a brake pressure corresponding to the desired braking effect. For this purpose, a brake line originating from the control valve is usually connected with a brake cylinder at the vehicle wheel, which brake cylinder generates the braking force required for the braking of the vehicle wheel for a disk or drum brake connected thereto. The defining of the desired pressure value for the electropneumatic control valve can take place electrically as well as pneumatically. However, the pneumatic control is mostly utilized only for safety purposes in the event of a failure of the electric control.
An electropneumatic control valve of this type is known from German Patent Document DE 39 31 761 C2. The control valve has a large-surface control piston which is accommodated in an axially displaceable manner inside a cylindrical hollow space of a relay valve housing. The control piston separates the hollow space into a control chamber and into a working chamber. In the case of the working chamber, a coaxial hollow pin is constructed on the control piston, which hollow pin is used for actuating an adjacent valve seat arrangement. By means of an axial adjustment of a spring-braced valve tube, the valve seat arrangement switches the compressed-air flow between an exterior brake line connection, a supply pressure connection as well as a bleeder connection.
By means of a pilot valve arrangement, the control piston can be acted upon by a control pressure on the control chamber. The pilot valve arrangement consists of two electropneumatic pilot valves which, by means of a coordinated energizing of the integrated electric coils, cause a rising, maintaining or lowering of the control pressure within the control chamber in order to implement the desired movement of the control piston while interacting with the restoring spring arranged on the opposite side of the control piston.
The axial guidance required because of the large-surface characteristic of the control piston is achieved here by way of a coaxial guide bore in the pilot valve housing, which guide bore corresponds with a guiding pin coaxially shaped to the control piston.
However, this type of guidance has the disadvantage that the guide bore in the pilot valve housing must be produced in a highly precise manner and with a high surface quality in order to avoid a jamming or tilting of the control piston within the cylindrical hollow space during the operation. Furthermore, this known guiding arrangement causes a relatively complicated shaping of the control piston whose production therefore requires high expenditures. This applies particularly also to the making of the passage bore which extends coaxially through the control piston and is used for discharging outgoing air from the control chamber to the bleeder connection. Here, the passage bore has a relatively large diameter in order to permit a fast bleeding of the control chamber.
In practice, it has been attempted to design a guiding arrangement for the control piston according to another, constructively simpler principle, which is known from German Patent Document DE 196 05 562 A1. In that case, a sleeve is used for the guidance, which sleeve is joined into a suitable coaxial recess of the pilot valve. By way of the outside diameter of the sleeve projecting from the pilot valve housing in this manner, the control piston is guided by way of a corresponding coaxial passage bore for this purpose.
The fastening of the sleeve on the pilot valve housing takes place with a radial play and/or swivel play for the compensation of a production-caused play between the control piston and the pilot valve housing. However, this type of guidance of the control piston results in a deterioration of the guiding characteristics because a tilting of the control piston becomes possible as a result of the swivel play and/or radial play.
Furthermore, the guiding principle applied here requires a small space for the passage bore for discharging the outgoing air from the control chamber. To this extent, only a comparatively small effective cross-section is possible.
It is therefore an object of the present invention to improve an electropneumatic control valve of the above-described type such that a precise guiding of the control piston can be carried out with an effective integrated guiding of outgoing air at low manufacturing expenditures.
The object is achieved by an electropneumatic control valve of the present invention having a relay valve housing in which an axially adjustable control piston is arranged for actuating a valve seat arrangement which switches the compressed-air flow between at least one exterior brake line connection, a supply pressure connection and a bleeder connection. The control piston is axially guided by means of a coaxial guiding arrangement and capable of being acted upon by a control pressure by way of at least one electromagnetic pilot valve for the axial adjustment. The pilot valve is accommodated in a pilot valve housing connected with the relay valve housing. The guiding arrangement of the control piston comprises a separate guiding sleeve which is stationarily fastened to the pilot valve housing and which is provided with an upper guiding section which, for the axial guidance, corresponds with a passage bore in the control piston, and which, in addition, has a lower tube section of a smaller outside diameter adjoining the upper guiding section in order to guide the outgoing air flowing through an integrated axial bleeder duct over the area of the valve seat arrangement directly to the bleeder connection.
The invention includes the technical teaching that the guiding arrangement of the control piston consists of a separate guiding sleeve which is fastened to the pilot valve housing and which is provided with an upper guiding section which, for the axial guidance, corresponds with a passage bore in the control piston and which, in addition, has a lower tube section of a smaller outside diameter which adjoins the upper guiding section in order to guide the outgoing air flowing through an integrated axial bleeder duct over the area of the valve seat arrangement directly to the bleeder connection.
The advantage of the invention is particularly the special further development of the guiding sleeve to be used, which provides a precise guiding function for the control piston as well as an effective outgoing-air guidance. Because of the particularly long bleeder duct which extends directly to the bleeder connection over the area of the valve seat arrangement, the opening of the bleeder duct arrives in an area of a lower ram pressure, whereby, because of the low existing counterpressure in the proximity of the bleeder connection, a bleeding of the control chamber is achieved which is faster despite the relative small diameter of the bleeder duct. Surprisingly, the tube section situated in the outgoing-air flow of the working chamber results in an extreme noise reduction during the bleeding of the brake line.
In order to further minimize the noise caused by the outflowing air at the electropneumatic control valve, a sound absorber can, in addition, be mounted on the bleeder connection. In cooperation with the sound absorber, the distal end of the tube section of the guiding sleeve has a fissured construction in order to ensure a free flowing-out of the outgoing air from the bleeder duct. As a result of this arrangement, a very low noise level is achieved on the whole which meets even strict requirements with respect to a control valve of the above-mentioned type.
Preferably, the guiding sleeve according to the invention, at least in the area of the guiding section, consists of a plastic material with a low sliding friction coefficient. As a result, a low sliding friction is achieved which is favorable for the axial guidance because the control piston, as a friction partner, preferably is also produced of a plastic material. It is also conceivable to use different pairings of material. The corresponding passage bore in the control piston, to this extent, can also be equipped with a Teflon bush or the like.
According to another embodiment of the invention, a guide pin formed in the pilot valve housing is provided for fastening the guiding sleeve, in which case, by way of a force-locking press fit, a connection with the guiding sleeve is established. Corresponding with the guide pin, in the area of the guiding section, the guiding sleeve preferably has a cylindrical recess of a larger diameter, which extends essentially coaxially with respect to the bleeder duct, by means of which recess, the fastening takes place by a pressing onto the guide pin. Since it is sufficient in this case to provide only two axially spaced, ring-shaped fitting sections inside the recess, which fitting sections come to rest on the guide pin, because of the minimized pressing-on force, this pressing-on can also take place manually. In addition, the two spaced ring-shaped fitting sections ensure a precise positioning of the guiding sleeve with respect to the guide pin.
According to another advantageous embodiment of the guiding sleeve, a step is constructed in the distal end area of the tube section in order to permit an end-side form-locking fixing of the guiding sleeve in a stationary manner with respect to the relay valve housing. As a result, despite the large length, the guiding sleeve is securely fixed inside the relay valve housing at both ends, specifically in the upper area of the recess as well as in the lower area of the opening of the bleeder duct.
In order to permit a particularly fast flowing of the outgoing air out of the bleeder duct, it is advantageous for the flow cross-section of the bleeder duct to be maximal. However, a minimal required wall thickness of the tube section should be retained in order to ensure a sufficient stability of the tube section. The outside diameter of the tube section, in turn, is limited by a minimally required ring gap for the outgoing air from the brake line when the latter is bled. In order to meet all these requirements, the wall of the tube section can additionally be reinforced by a ring arrangement, so that the required wall thickness as well as the overall diameter of the tube section can be kept as small as possible and a sufficient stability is nevertheless achieved. The rib arrangement preferably consists of several shaped-on ribs extending axially along the outside wall of the tube section. The shaped-on ribs are aligned essentially axially parallel with respect to the tube section and, in the cross-sectional view, are arranged at an equal distance from one another. The result is therefore a star-type shape in a cross-sectional view.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.